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JavaScript The JavaScript 'use strict' Statement

Posted by Sarah Saad
Sarah Saad
Courses Plus Student 3,223 Points

'Strict Mode' catches the mistyped variable names in an assignment. How does the JavaScript Engine works it out?

As explained here https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Strict_mode

First, strict mode makes it impossible to accidentally create global variables. In normal JavaScript mistyping a variable in an assignment creates a new property on the global object and continues to "work" (although future failure is possible: likely, in modern JavaScript).

2 Answers

Steven Parker
Steven Parker
231,846 Points
Steven Parker
Steven Parker
231,846 Points

It doesn't actually detect that you mistyped anything. It just does not allow a variable to be created implicitly.

jsdevtom
jsdevtom
16,963 Points
jsdevtom
jsdevtom
16,963 Points

I'll start off by saying that I am no expert. Therefore, I'll include how I found this information so that you can see if I made any mistakes along the way.

After some googling, I found out that javascript engines have a parser¹. This guide to Javascript engines is pretty informative as well. Then I went to the v8 engine's repo on github. I then went to have a look at the parser.cc file where I found this section on checking if the code is in use strict mode.

I don't think you have to understand c++ to understand what is going on here. The code checks if there is a directive 

 if (directive_prologue) {

After which it checks if the 'use strict'; statement is there:

// Check "use strict" directive (ES5 14.1), "use asm" directive.
        bool use_strict_found =

If use_strict_found (and some other weird condition) evaluates as true , it changes the language mode:

RaiseLanguageMode(STRICT);

EDIT: Oh, I read the question wrongly. I don't know. I do know that after the parser has determined that you are using strict mode, the parser sets a different set of semantics from the normal code². AKA, ( I guess ) it will do something to this 'When var is declared, check it has a var sign in front of it. False?: Check if it is declared in a higher scope. False: check if strict mode is enabled. If it is, throw error. Again this is just speculation. I couldn't the relevant section of the code down.

  1. How does a Javascript engine work

  2. "Strict mode isn't just a subset: it intentionally has different semantics from normal code." Source: Mozilla's strict mode reference