strict mode:

It is used to catch or throw errors when some unsafe actions are taken and  use strict directive is used to execute javascript code in strict mode. For example: we cannot use undeclared variable in strict mode.

use strict can be used only at the beginning of a function  or of a script.

How to declare strict mode:

If we write use strict inside a function then only that function will execute in strict mode and if we have added in first line of script then whole script will execute in strict mode.

Example 1:

<script>
x = 21;        // This will not cause an error.
myFunc();

function myFunc() {
    "use strict";
    y = 21;    /* This will cause an error (y is not defined) as we have used strict mode in this function.*/
}
</script>

As in the above script we used use strict inside myFunc()  which means only myFunc() code  will execute in strict mode.

Example 2:

<script>
"use strict";
myFunc();

function myFunc() {
    y = 21;   // This will cause an error (y is not defined)
}
</script>

As in the above script we used use strict in the first line of script which means the whole script will execute in strict mode.