'this' value in Function Execution Context is related to Strict Mode

[young-trigold]

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Edition: 2nd

Book Title: get started

Chapter: 3

Section Title: this keyword

Question: I think this section misses a subtle but fatal part: strict mode.

"this" value in Standard Function (not arrow function) Execution Context is related to Strict Mode.

this value in a function directly invoked in strict mode is undefined, if not, global object.

The code sample:

var log = console.log;

var thisValue1 = (function getThisInStrict() {
  'use strict';
  return this;
})();

var thisValue2 = (function getThis() {
  return this;
})();

log(thisValue1 === thisValue2);
// -> false

I hope this section can add some materials on arrow function, execution context, Function.prototype.call().

hopefully helpful.

[getify]

Since this program is not in strict mode (see Chapter 1, "Strictly Speaking"), context-aware functions that are called without any context specified default the context to the global object (window in the browser).