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JavaScript - Math.random() method example

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Created by:

Aisha

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The Math.random() function returns floating-point, pseudo-random number between range [0,1), 0 (inclusive) and 1 (exclusive). Based on this function we are able to get a random number in range as we can see in the below examples.

Quick solutions:

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console.log( Math.random() );  // 0.9100486004606172
console.log( Math.random() );  // 0.8630710741089413
console.log( Math.random() );  // 0.8052253695967542

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Alternative solution:

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function randomInt(min, max) {
    return min + Math.floor((max - min) * Math.random());
}
​
console.log( randomInt( 10, 20) );  // 12
console.log( randomInt(-10, 10) );  // -4

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function randomFloat(min, max) {
    return min + (max - min) * Math.random();
}
​
console.log( randomFloat( 10, 20) );  // 14.514897223860018
console.log( randomFloat(-10, 10) );  // -6.645075993092653

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1. Documentation

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Syntax	`Math.random()`
Parameters	This method does not take any arguments.
Result	Float `number` value (primitive value).
Description	`random` is a static method that returns a random float number from the range `<0, 1)` - inclusive 0 and exclusive 1.

2. Custom random method examples

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2.1. Random float in range example (exclusive max value)

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This example shows how to random numbers with exclusive max. randomizeFloat() method is overridden in the following ways:

randomizeFloat() - generates numbers in a range from 0 to Number.MAX_VALUE (exclusive),
randomizeFloat(max) - generate numbers in a range from 0 to max (exclusive) - max value must be positive,
randomizeFloat(min, max) - generates numbers in a range from min to max (exclusive).

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/*
    inclusive min (result can be equal to min value)
    exclusive max (result won't be equal to max value)
*/
function randomizeFloat(min, max) {
    if (max == null) {
        if (min <= 0) {
            throw new Error('Max value must be positive.');
        }
        max = (min == null ? Number.MAX_VALUE : min);
        min = 0.0;
    }
    if (min >= max) {
        throw new Error("Incorrect arguments.");
    }
    return min + (max - min) * Math.random();
}
​
​
// Usage example:                      // Example output:
​
console.log(randomizeFloat());         //   1.67319916301163e+308
console.log(randomizeFloat(5));        //   2.7593705936801918
console.log(randomizeFloat(10, 80));   //  37.54521514384005
console.log(randomizeFloat(-50, 50));  // -30.632843429520975

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2.2. Random float in range example (inclusive max value)

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// Generates values from <0, 1>
//
function randomizeNumber() {
    return Math.min(1, 1.0000000000000003 * Math.random());
}
​
/*
    inclusive min (result can be equal to min value)
    inclusive max (result can be equal to min value)
*/
function randomizeFloat(min, max) {
    if(max == null) {
        max = (min == null ? Number.MAX_VALUE : min);
        min = 0.0;
    }
    if(min >= max) {
        throw new Error("Incorrect arguments.");
    }
    return min + (max - min) * randomizeNumber();
}
​
​
// Usage example:                      // Example output:
​
console.log(randomizeFloat());         //   1.1960373039711962e+308
console.log(randomizeFloat(5));        //   0.7663988388633522
console.log(randomizeFloat(10, 80));   //  67.81113931017913
console.log(randomizeFloat(-50, 50));  // -13.713816892801674

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2.3. Random integer in range example (exclusive max value)

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/*
    inclusive min (result can be equal to min value)
    exclusive max (result won't be equal to max value)
*/
function randomizeInteger(min, max) {
    if(max == null) {
        max = (min == null ? Number.MAX_SAFE_INTEGER : min);
        min = 0;
    }
​
    min = Math.ceil(min);  // inclusive min
    max = Math.floor(max); // exclusive max
​
    if(min > max - 1) {
        throw new Error("Incorrect arguments.");
    }
​
    return min + Math.floor((max - min) * Math.random());
}
​
// Example usage:                        // Example output:
​
console.log(randomizeInteger());         //  5547382624322139
console.log(randomizeInteger(5));        //  3
console.log(randomizeInteger(10, 80));   //  62
console.log(randomizeInteger(-50, 50));  // -8

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2.4. Random integer in range example (inclusive max value)

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/*
    inclusive min (result can be equal to min value)
    inclusive max (result can be equal to min value)
*/
function randomizeInteger(min, max) {
    if(max == null) {
        max = (min == null ? Number.MAX_SAFE_INTEGER : min);
        min = 0;
    }
    min = Math.ceil(min);   // inclusive min
    max = Math.floor(max);  // exclusive max
    if(min > max - 1) {
        throw new Error("Incorrect arguments.");
    }
    return min + Math.floor((max - min + 1) * Math.random());
}
​
​
// Example usage:                        // Example output:
​
console.log(randomizeInteger());         //  5918572174489812
console.log(randomizeInteger(5));        //  5
console.log(randomizeInteger(10, 80));   //  60
console.log(randomizeInteger(-50, 50));  // -15