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JavaScript - custom simple template engine

christachrista
600
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// Based on: https://dirask.com/snippets/JavaScript-custom-name-builder-allowed-a-z-A-Z-and-0-9-characters-DnYvq1
//
const PATTERNS_GUARDIAN = new Set(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']);

// Source: https://dirask.com/snippets/JavaScript-check-if-value-is-collection-Array-Set-Map-etc-jQA7rj
//
const isCollectionValue = (value) => {
    if (value == null) {  // null or undefined
        return false;
    }
    return !!value[Symbol.iterator];
};

// Checks if value can not be used.
//
const isNegativeValue = (value) => {
    if (value == null) {  // null or undefined
        return true;
    }
    if (value.length === 0 || value.size === 0) {
        return true;
    }
    return false;
};

// Checks if value can be used.
//
const isPositiveValue = (value) => {
    if (value == null) {  // null or undefined
        return false;
    }
    if (value.length === 0 || value.size === 0) {
        return false;
    }
    return true;
};

// Based on: https://dirask.com/snippets/JavaScript-custom-name-builder-allowed-a-z-A-Z-and-0-9-characters-DnYvq1
//
function NameBuilder() {
    let _text = '';
    this.append = (character) => {
        if (PATTERNS_GUARDIAN.has(character)) {
            _text += character;
        } else {
            throw new Error('Name syntax error.');
        }
    };
    this.extract = () => {
        const result = _text;
        if (result === '') {
            throw new Error('Name syntax error.');
        }
        _text = '';
        return result;
    };
}

// Based on: https://dirask.com/snippets/JavaScript-custom-path-builder-allowed-a-z-A-Z-and-0-9-characters-in-names-and-dot-as-separator-p2KV8j
//
function PathBuilder() {
    const _builder = new NameBuilder();
    let _parts = new Array();
    this.append = (character) => {
        _builder.append(character);
    };
    this.separate = () => {
        _parts.push(_builder.extract());
    };
    this.extract = () => {
        _parts.push(_builder.extract());
        return _parts.splice(0);
    };
}

function TextTemplate(_config) {
    if (_config == null) {  // null or undefined
        throw new Error('Configuration is not provided.');
    }
    const _template = _config.template;
    const _filters = _config.filters;
    const _negativeChecker = _config.negativeChecker || isNegativeValue;  // OR:  _config.negativeChecker ?? isNegativeValue
    const _positiveChecker = _config.positiveChecker || isPositiveValue;  // OR:  _config.positiveChecker || isPositiveValue
    if (_template == null) {  // null or undefined
        throw new Error('Template is not provided.');
    }
    if (_filters == null) {  // null or undefined
        throw new Error('Filters are not provided.');
    }
    const _root = {
        parent: null,
        levels: new Array()
    };
    let _level = _root;
    let _index = 0;
    let _part = '';
    const _readName = (limiter) => {
        const builder = new NameBuilder();
        while (true) {
            if (_index < _template.length) {
                const entry = _template[_index++];
                try {
                    switch (entry) {
                        case limiter:
                            return builder.extract();
                        default:
                            builder.append(entry);
                            break;
                    }
                } catch (e) {
                    throw new Error(`Name syntax error at position ${_index}.`);
                }
            } else {
                throw new Error(`Template syntax error at position ${_index}.`);
            }
        }
    };
    const _readPath = (limiter) => {
        const builder = new PathBuilder();
        while (true) {
            if (_index < _template.length) {
                const entry = _template[_index++];
                try {
                    switch (entry) {
                        case '.':
                            builder.separate();
                            break;
                        case limiter:
                            return builder.extract();
                        default:
                            builder.append(entry);
                            break;
                    }
                } catch (e) {
                    throw new Error(`Path syntax error at position ${_index}.`);
                }
            } else {
                throw new Error(`Template syntax error at position ${_index}.`);
            }
        }
    };
    const _readPipe = (limiter) => {
        const builder = new PathBuilder();
        while (true) {
            if (_index < _template.length) {
                const entry = _template[_index++];
                try {
                    switch (entry) {
                        case '.':
                            builder.separate();
                            break;
                        case '|':
                            const filter = _readName(limiter);
                            return {
                                filter,
                                path: builder.extract()
                            };
                        case limiter:
                            return {
                                path: builder.extract()
                            };
                        default:
                            builder.append(entry);
                            break;
                    }
                } catch (e) {
                    throw new Error(`Pipe syntax error at position ${_index}.`);
                }
            } else {
                throw new Error(`Template syntax error at position ${_index}.`);
            }
        }
    };
    const _findValue = (level, proxy) => {
        const path = level.path;
        switch (path.length) {
            case 1:
                level = level.parent;
                if (level) {
                    const mixed = path[0];
                    while (true) {
                        if (mixed === level.base) {
                            const data = proxy.data;
                            return data == null ? null : data;  // null or undefined  // OR:  value ?? null
                        }
                        level = level.parent;
                        if (level == null) {  // null or undefined
                            break;
                        }
                        proxy = proxy.parent;
                    }
                    const value = proxy.data[mixed];
                    return value == null ? null : value;  // null or undefined  // OR:  value ?? null
                }
                return null;
            case 2:
                level = level.parent;
                if (level) {
                    const major = path[0];
                    const minor = path[1];
                    while (true) {
                        if (major === level.base) {
                            const value = proxy.data[minor];
                            return value == null ? null : value;  // null or undefined  // OR:  value ?? null
                        }
                        level = level.parent;
                        if (level == null) {  // null or undefined
                            break;
                        }
                        proxy = proxy.parent;
                    }
                }
                return null;
            default:
                throw new Error('Template rendering error.');
        }
    };
    while (_index < _template.length) {
        const entry = _template[_index++];
        switch (entry) {
            case '{':
                const levels = _level.levels;
                levels.push({
                    type: 1,
                    text: _part
                });
                _part = '';
                if (_index < _template.length) {
                    const entry = _template[_index++];
                    switch (entry) {
                        case '=':
                            {
                                const pipe = _readPipe('}');
                                levels.push({
                                    ...pipe,
                                    type: 2,
                                    parent: _level
                                });
                            }
                            break;
                        case '+':
                            {
                                const path = _readPath('}');
                                _level = {
                                    path,
                                    type: 3,
                                    parent: _level,
                                    levels: new Array()
                                };
                                levels.push(_level);
                            }
                            break;
                        case '-':
                            {
                                const path = _readPath('}');
                                _level = {
                                    path,
                                    type: 4,
                                    parent: _level,
                                    levels: new Array()
                                };
                                levels.push(_level);
                            }
                            break;
                        case '#':
                            {
                                const path = _readPath(':');
                                const base = _readName('}');
                                _level = {
                                    path,
                                    base,
                                    type: 5,
                                    parent: _level,
                                    levels: new Array()
                                };
                                levels.push(_level);
                            }
                            break;
                        case '/':
                            {
                                if (_index < _template.length) {
                                    const entry = _template[_index++];
                                    if (entry !== '}') {
                                        throw new Error(`Template syntax error at position ${_index}.`);
                                    }
                                }
                                _level = _level.parent;
                                if (_level == null) {  // null or undefined
                                    throw new Error(`Template syntax error at position ${_index}.`);
                                }
                            }
                            break;
                        default:
                            throw new Error(`Template syntax error at position ${_index}.`);
                    }
                } else {
                    throw new Error(`Template syntax error at position ${_index}.`);
                }
                break;
            case '}':
                throw new Error(`Template syntax error at position ${_index}.`);
            case '\\':
                if (_index < _template.length) {
                    const entry = _template[_index++];
                    switch (entry) {
                        case '{':
                            _part += '{';
                            break;
                        case '}':
                            _part += '}';
                            break;
                        case '\\':
                            _part += '\\';
                            break;
                        default:
                            throw new Error(`Template syntax error at position ${_index}.`);
                    }
                } else {
                    throw new Error(`Template syntax error at position ${_index}.`);
                }
                break;
            default:
                _part += entry;
                break;
        }
    }
    if (_root !== _level){
        throw new Error(`Template syntax error at position ${_index}.`);
    }
    _level.levels.push({
        type: 1,
        text: _part
    });
    this.render = (...data) => {
        let result = '';
        const process = (level, proxy) => {
            for (const entry of level.levels) {
                switch (entry.type) {
                    case 1:
                        result += entry.text;
                        break;
                    case 2:
                        {
                            const filter = entry.filter;
                            if (filter == null) {  // null or undefined
                                result += _findValue(entry, proxy);
                            } else {
                                const action = _filters[filter];
                                if (action == null) {  // null or undefined
                                    throw new Error(`'${filter}' filter is not provided.`);
                                }
                                const value = _findValue(entry, proxy);
                                result += action.call(null, value);
                            }
                        }
                        break;
                    case 3:
                        {
                            const value = _findValue(entry, proxy);
                            if (_positiveChecker.call(null, value)) {
                                const wrapper = {
                                    parent: proxy
                                };
                                process.call(null, entry, wrapper);
                            }
                        }
                        break;
                    case 4:
                        {
                            const value = _findValue(entry, proxy);
                            if (_negativeChecker.call(null, value)) {
                                const wrapper = {
                                    parent: proxy
                                };
                                process.call(null, entry, wrapper);
                            }
                        }
                        break;
                    case 5:
                        {
                            const value = _findValue(entry, proxy);
                            if (isCollectionValue(value)) {
                                for (const data of value) {
                                    const wrapper = {
                                        data,
                                        parent: proxy
                                    };
                                    process.call(null, entry, wrapper);
                                }
                            }
                        }
                        break;
                    default:
                        throw new Error('Template rendering error.');
                }
            }
        };
        for (const entry of data) {
            const wrapper = {
                data: entry
            };
            process.call(null, _root, wrapper);
        }
        return result;
    };
}



// Usage example:

const hCase1 = document.querySelector('#case-1');
const hCase2 = document.querySelector('#case-2');

const template = new TextTemplate({
    template: `
        <p>Hi {=name|upper}!</p>
        <p>
          {+items}
            Your items are:
            <ul>
              {#items:item}
                <li>
                  {=item.name|lower} ({=item.count})
                </li>
              {/}
            </ul>
          {/}
          {-items}
            You don't have items!
          {/}
        </p>
    `,
    filters: {
        upper: (value) => {
            return value.toUpperCase();
        },
        lower: (value) => {
            return value.toLowerCase();
        }
    }
});

const result1 = template.render({
    name: 'John',
    items: [
        {name: 'Book',   count: 2},
        {name: 'Pencil', count: 4},
        {name: 'Marker', count: 1}
    ]
});
const result2 = template.render({
    name: 'Kate',
});

console.log(result1);
console.log(result2);
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