JavaScript - What is strict mode?

Strict mode in JS means strict operating context. It can be enabled by putting ‘use strict’ at the top of the file or at the top of a function.

'use strict';
//other functions and variables

//To enable only for a particular function
function(param){
  'use strict';
  //Now only this function is in strict mode
}

Strict mode has the following effect on the code execution:

Variable can not be used without declaration.

(() => {
  'use strict';
  undeclaredVar = 'someValue'; //Not allowed
  //Error: undeclaredVar is not defined
})();

Not possible to use reserved keywords as variable names. e.g. let, const, var etc.

(() => {
  'use strict';
  var eval = 'someValue'; //Not allowed
  //Unexpected strict mode reserved word
})();

delete of var, function and function arguments is not allowed.

(() => {
  'use strict';
  var toBeDeleted = 'TryDeletingMe';
  var aFunction = () => 'JustString';
  delete toBeDeleted; // Not allowed
  delete aFunction; // Not allowed
  //Delete of an unqualified identifier in strict mode.
  
})();

variables defined inside eval are not available outside of the eval.

(() => {
  'use strict';
  eval('var varInEval = 12;');
  console.log(varInEval);
  //Error: varInEval is not defined
  //In non-strict mode it would have printed 12
})();

The default value of this is undefined however in non-strict mode it is window object.

(() => {
  'use strict';
  console.log(this); //prints undefined
  //In non-strict mode it prints global window object
})();

Duplicating parameter in a function is not allowed

(() => {
  'use strict';
  //Not allowed
  function withDuplicateParamName(param, param) {
    console.log(param);
  }
  //Duplicate parameter name not allowed in this context
})();

It is always recommended to use strict mode.