The “use strict” directive in JavaScript was introduced in ES5 to help you catch common coding mistakes and “unsafe” actions like assigning values to read-only variables or using some of the reserved keywords.

When you put “use strict” at the top of your JavaScript code or function, the JavaScript interpreter will switch to the Strict Mode, and it will start to catch and throw errors for actions that would be ignored or would fail silently in normal, non-strict code.

Strict mode makes several changes to the normal JavaScript behavior, such as turning some silent errors into thrown errors, fixing mistakes that can make it difficult for JavaScript engines to perform optimizations, and prohibiting certain syntax that's likely to be defined in future versions of ECMAScript.

It must be enclosed in quotes, either single ('use strict') or double (“use strict”).

"use strict";
 
function sayHello() {
  console.log(this.name, this.age);
}
 
// rest of the code

function sayHello() {
  "use strict";
  console.log(this.name, this.age);
}
 
// rest of the code

"use strict";
 
function sayHello() {
  console.log(this.name, this.age);
}
 
// ... rest of the code ...
 
sayHello(); // TypeError: Cannot read properties of undefined (reading 'name')

In strict mode, this inside the sayHello function will be undefined when called without an object, leading to a TypeError when attempting to access properties on it.

"use strict";
x = 3.14; // This will throw an error because x is not declared.

Assigning to Read-Only Global Variables and Properties: Assigning a value to a read-only property, a getter-only property, a non-writable global variable, or a non-writable property of an object will throw an error.

"use strict";
undefined = "test"; // This will throw an error

Deleting Variables, Functions, or Function Parameters: Attempting to delete variables, functions, or function parameters will throw an error.

"use strict";
function x(){}
delete x; // This will throw an error

Duplicate Parameter Names: Duplicate parameter names in function declarations will throw an error.

"use strict";
function x(p1, p1) {} // This will throw an error

Octal Syntax: Octal syntax in ECMAScript 5 is deprecated, and strict mode will not allow it.

"use strict";
var x = 010; // This will throw an error