JavaScript Introduction | Exception Handling | Strict Mode

What Is Strict Mode?

Strict mode, first introduced in ECMAScript 5, applies stricter error checking to JavaScript code.

Strict mode can be declared by using the "use strict" directive at the beginning of a script or function.

"use strict"    // Set the entire script to strict mode.
try {
    num = 3.14; // Causes an error because an undeclared variable is used.
} catch (ex) {
    document.getElementById("text").innerHTML = ex.name + "<br>";
    document.getElementById("text").innerHTML += ex.message;
}

Strict mode declared this way strictly checks the code in the corresponding block according to strict mode syntax.

str = "Mistake!";    // An undeclared variable is used, but it is automatically declared as a global variable.
document.getElementById("noStrict").innerHTML = str + "<br>";
function StrictBlock() {
    "use strict"  // Set only this function block to strict mode.
    try {
        num = 123 // Causes an error because an undeclared variable is used.
    } catch (ex) {
        document.getElementById("funcStrict").innerHTML = ex.name + "<br>";
        document.getElementById("funcStrict").innerHTML += ex.message;
    }
}
StrictBlock();

In the global area that is not in strict mode in the example above, using an undeclared variable automatically treats it as a global variable. However, in a function block declared as strict mode, using an undeclared variable causes an error.

The major web browser versions that support strict mode are as follows.

Characteristics of Strict Mode

JavaScript strict mode uses syntax that restricts some features of the existing JavaScript language. It also modifies several important features to provide enhanced security along with stronger error checking.

The syntax of strict mode that differs from existing JavaScript syntax is as follows.