Scott Whittaker

Frontend Developer

Day job React | side projects Svelte

Discovering the identity function

I fixed a bug recently: TypeError: Cannot read property 'id' of undefined. This one was simple enough, I had created an array which had undefined values in it and later when iterating that array the expected value was not present, hence the error.

The issue

Here is a contrived example of the issue; there are 2 arrays, data1 and data2. We want to create a new array by iterating data1 and finding objects in data2 that have the same id.

const data1 = [{ id: '1' }, { id: '2' }, { id: '76' }, { id: '378' }, { id: '879' }];
const data2 = [{ id: '2' }, { id: '378' }];

// map returns an array with the same length as the array you are mapping over
const result = (item1) {
	// find will return undefined if the test is not satisfied
	return data2.find(function (item2) {
		return ===;

Or the ES6 approach…

const result = => data2.find((item2) => ===;

The resulting array contains 3 undefined entries and 2 objects, those objects whose id values matched. The reason we have undefined values in the array is firstly, when we map over an array it will always return a new array with same length as the array we are mapping over, hence data1.length is 5 and result.length is 5. Additionally we pass a function to map which uses Array.prototype.find() to determine if each item in the data1 array exists in the data2 array. find returns the value if it matched, otherwise it returns undefined.

[undefined, { id: '2' }, undefined, { id: '378' }, undefined];

Later in the code we iterate the result expecting each item in the array to be an object with an id property. The first value in the array is undefined and an error is thrown.

result.forEach((item) => console.log(; // TypeError: Cannot read property 'id' of undefined

To fix the bug we need to filter out the undefined values in the array:

const result = data1
	.map((item1) => data2.find((item2) => ===
	.filter((value) => value !== undefined);

Filtering out undefined values from the array fixes the bug and we can move on.

console.log(result); // [ { id: '2' }, { id: '378' } ]

A more functional approach

There is nothing at all wrong with the above fix but I had a niggling feeling that this common filtering approach could be handled in a more declarative way. Eventually I found the answer in the book Functional-Light JavaScript.

It turns out that in functional programming there is a common base utility function called identity which returns the argument that it receives.

Because identity(..) simply returns the value passed to it, JS coerces each value into either true or false, and that determines whether to keep or exclude each value in the final array.

Functional-Light JavaScript

For example the lodash library has an identity function.

function identity(value) {
	return value;

We can use the identity function as follows to filter the results.

const result = => data2.find((item2) => ===;

This yields the same results but the code is more declarative.

console.log(result); // [ { id: '2' }, { id: '378' } ]

It turns out that we can also use the built-in Boolean function to achieve the same thing.

Tip: Another unary function that can be used as the predicate in the previous example is JS’s built-in Boolean(..) function, which explicitly coerces a value to true or false.

Functional-Light JavaScript
const result = => data2.find((item2) => ===;

console.log(result); // [ { id: '2' }, { id: '378' } ]