Developer Notes

JavaScript: find the types of a variables

January 12, 2021

Often I need to find what is the type of given variable so I could decide how to handle it or what methods I may expect. At the same time using typeof is not always the solution we need. For example typeof [] will return object that is because arrays are extended objects but it’s not what we want to get.

There is Array.isArray([]) that will return boolean but again we need to check for multiple types.

Function below gonna returning string value of the passed variable:

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1function type(value) {
2  return Object.prototype.toString
3    .call(value)
4    .replace(/\[object |\]/gi,'')
5    .toLowerCase();
6}

I’m going with .toLowerCase() just to not relay on capital latters when comparing types. After all we are working with exact string matching later on.

Let’s write a test about it to be sure that everything is as expected:

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1describe('type', () => {
2  it('should return the correct type', () => {
3    [
4      ['map', new Map()],
5      ['set', new Set()],
6      ['weakmap', new WeakMap()],
7      ['object', {}],
8      ['string', 'Hello'],
9      ['array', []],
10      ['boolean', true],
11      ['number', 5],
12      ['function', () => {}],
13      ['date', new Date()],
14      ['generatorfunction', function* generator() {}],
15    ].forEach(([expectedType, value]) => {
16      expect(type(value)).toBe(expectedType)
17    })
18  })
19})

The test is not very extensiv or cover all posible cases but still - show that in most common cases it’s working.

In the cases where we need to accept for example two or more types, now there is a simple way of checking them without writing multiple if:

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1function oneOfType(value, types) {
2  return (types || []).includes(type(value))
3}
4
5const foo = 'must-be-string'
6
7if (oneOfType(foo, ['string', 'array'])) {
8  console.log(foo.length)
9}

My name is Bozhidar Dryanovski and I'm a Software Engineer at Clarity
You should follow me on Twitter or Check my work at Github

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