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JavaScript - how to calculate page number when we know size of page and item index?
5 points
In this article, we're going to have a look at how in JavaScript, calculate page number on which item is located, when we know:
- maximal size of the single page,
- item index (counted from
0).
Simple example:
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const calculatePageNumber = (pageSize, itemIndex) => {2
return Math.ceil(++itemIndex / pageSize);3
};4
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// Example usage:7
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const pageSize = 10;9
const itemIndex = 29;10
const pageNumber = calculatePageNumber(pageSize, itemIndex);11
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console.log('pageNumber=' + pageNumber); // pageNumber=3The above logic result explanation:
- the maximal size of the single page is
10(pageSizevariable),- the item index is
29(itemIndexvariable),So:
- page
1should contain item indexes from0to9- page
2should contain item indexes from10to19- page
3should contain item indexes from20to29so we have item on
3rd page.
In this section we present simple test for different calculatePageNumber() usage cases.
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const calculatePageNumber = (pageSize, itemIndex) => {2
return Math.ceil(++itemIndex / pageSize);3
};4
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// Exmaple tests:7
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const pageSize = 10;9
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var testingData = [11
{ itemIndex: 0, pageNumber: 1 },12
{ itemIndex: 1, pageNumber: 1 },13
{ itemIndex: 9, pageNumber: 1 },14
{ itemIndex: 10, pageNumber: 2 },15
{ itemIndex: 11, pageNumber: 2 }16
];17
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console.log('Page numbers:');19
console.log('[expected]\t[actual]');20
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for (const entry of testingData) {22
const pageNumber = calculatePageNumber(pageSize, entry.itemIndex);23
if (entry.pageNumber !== pageNumber) {24
throw new Error('Page number calculator error.');25
}26
console.log(entry.pageNumber + '\t\t' + pageNumber); // expected vs actual27
}
