ContextQMD
Back to Developer Roadmap

Strict Mode

src/data/question-groups/javascript/content/strict-mode.md

4.0835 B
Original Source

Advanced concepts

JavaScript's strict mode makes you follow strict rules (best practices) when writing code. It prevents common mistakes JavaScript often ignores, such as undeclared variables, and keeps your code safe. Add the string "use strict"; at the top of the JavaScript file or function to enable strict mode.

javascript
// Without strict mode

courseNo = "200";

console.log(courseNo); // 200

// With strict mode

"use strict";  // top of your JS file

courseNo= "200";

console.log(courseNo); // courseNo is not defined i.e, you didn't create it using the var, const, or let keywords. 


// Strict mode: Functions

function strictExample() { 

"use strict"; 

courseNumber = "200"; //  Error: courseNumber is not defined

console.log(courseNumber ); // code won't run

} 

strictExample(); // ReferenceError