`this` inside callback to forEach

class Counter {
  constructor() {
    this.sum = 0
  }

  add(arr) {
    arr.forEach(function (item) {
      this.sum += item // <-- (this === undefined)
    })
  }
  
  show() {
    console.log(this.sum)
  }
}

const counter = new Counter()
counter.add([1, 2, 3]) 
counter.show()

add function above will cause following TypeError as this inside the anonymous callback function is undefined.

TypeError: Cannot read properties of undefined (reading 'sum')

This is because the callback function passed to forEach creates its own scope. It makes this inside the callback (this === undefined) and outside the callback (this has access to sum: this.sum) different.

How to fix (bind) this to callback function inside forEach:

Fix 1: optional argument to forEach

add(arr) {
  arr.forEach(function (item) {
    this.sum += item
  }, this) // <--
}

Array.prototype.forEach()

Syntax

forEach(callbackFn, thisArg)

thisArg (Optional): A value to use as this when executing callbackFn.

Fix 2: closure

add(arr) {
  const that = this // <--
  arr.forEach(function (item) {
    that.sum += item // <-- that, not this
  })
}

Closures

Lexical context has changed inside the callback function. The callback function still has access to variables in their enclosing scope (add(){...}) throuth closure.

add has access to this
callback doesn't have direct access to this
but callback has access to variables inside add
copy this inside add so callback can access this

Fix 3: bind

add(arr) {
  arr.forEach(
    function (item) {
      this.sum += item
    }.bind(this) // <--
  )
}

Function.prototype.bind()

Explicitly set the value of this within the callback function.

Excerpt from mdn: this

The bind() method can set the value of a function's this regardless of how it's called.

Fix 4: arrow function

add(arr) {
  arr.forEach((item) => {
    this.sum += item;
  });
}

Excerpt from mdn: this

Arrow functions don't provide their own this binding (it retains the this value of the enclosing lexical context).

this

this inside callback to forEach