Concept 1
Strict Mode
Resouce: Strict mode
“use strict”
The “use strict” directive was new in ECMAScript version 5.
The purpose of “use strict” is to indicate that the code should be executed in “strict mode”.
Changes
Strict mode makes several changes to normal JavaScript semantics:
Eliminates some JavaScript silent errors by changing them to throw errors. Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes be made to run faster than identical code that’s not strict mode. Prohibits some syntax likely to be defined in future versions of ECMAScript.
Examples
"use strict";
x = 3.14; // This will cause an error because x is not declared
x = 3.14; // This will not cause an error.
myFunction();
function myFunction() {
"use strict";
y = 3.14; // This will cause an error
}