JavaScript - strict mode

In this article, we would like to explain what is strict mode in JavaScript.

1. Declaring

We declare strict mode with "use strict"; directive at the beginning of:

• a function - so only code in the function will be executed in the strict mode (local scope),
• a script - so all the code in the script will be executed in the strict mode (global scope).

The "use strict" is a literal expression that tells JavaScript to execute code in a strict mode. It was presented in ES5 (ignored by earlier versions of JavaScript).

2. What strict mode does?

Here are some main functionalities of the strict mode:

• it changes previously accepted "wrong syntax" into errors,
• it doesn't allow mistyping a variable name (which outside the strict mode creates a new global variable),
• any assignment to a non-writable / getter-only / non-existing property or non-existing variable/object will throw an error,
• this keyword in functions behaves differently, it refers to the object that called the function,
• basically, strict mode makes it easier to write secure code in JavaScript.

3. Things you can't do inside strict mode

• use variable or object without declaring it,
• delete variable, object or function (delete operator),
• duplicate parameter name,
• use octal numeric literals,
• use octal escape characters,
• delete undeletable property (like e.g Array.prototype),
• use eval and arguments keywords as the variable name,
• use with statement.

4. Examples

Using a variable without declaring it:

• without strict mode

  // ONLINE-RUNNER:browser;
  x = 10;  // No errors, we create new global variable
  console.log(x);

• with strict mode

  // ONLINE-RUNNER:browser;
  "use strict";
  x = 10;  // In strict mode this causes an error
  console.log(x);

Using a variable without declaring it inside a function:

• // ONLINE-RUNNER:browser;
  
  x = 10;  // This doesn't lead to an error
  console.log(x);
  
  foo();
  
  function foo() {
    "use strict";
    y = 20;  // This leads to an error
  }
  
  console.log(y);

References

• Strict mode - JavaScript | MDN