Strict mode is declared by adding "use strict"; to the beginning of a script or a function.
Declared at the beginning of a script, it has global scope (all code in the script will execute in strict mode):
Example
"use strict";
x = 3.14; // This will cause an error because x is not declared
x = 3.14; // This will cause an error because x is not declared
Example
"use strict";
myFunction();
function myFunction() {
y = 3.14; // This will also cause an error because y is not declared
}
myFunction();
function myFunction() {
y = 3.14; // This will also cause an error because y is not declared
}
Declared inside a function, it has local scope (only the code inside the function is in strict mode):
x = 3.14; // This will not cause an error.
myFunction();
function myFunction() {
"use strict";
y = 3.14; // This will cause an error
}
myFunction();
function myFunction() {
"use strict";
y = 3.14; // This will cause an error
}
The "use strict"; Syntax
The syntax, for declaring strict mode, was designed to be compatible with older versions of JavaScript.
Compiling a numeric literal (4 + 5;) or a string literal ("John Doe";) in a JavaScript program has no side effects. It simply compiles to a non existing variable and dies.
So "use strict"; only matters to new compilers that "understand" the meaning of it.
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