JavaScript Basics & Syntax Quiz

Strict Mode (“use strict”)

Strict Mode: JavaScript’s Safety Net

Strict mode is a way to opt into a restricted variant of JavaScript that catches common coding mistakes and prevents certain actions that might be confusing or ill-advised. It’s like turning on spell-check for your code.

Enabling Strict Mode

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What Strict Mode Prevents

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'use strict';

// 1. Prevents accidental global variables
// Without strict mode: this creates a global variable
// With strict mode: this throws an error
// undeclaredVar = 'This would be global';

// 2. Prevents duplicate parameter names
// function duplicateParams(a, a) { // Error in strict mode
//   return a;
// }

// 3. Prevents octal literals
// let octal = 010; // Error in strict mode
let octal = 0o10; // Use 0o prefix instead
console.log(octal); // 8

// 4. Prevents deleting undeletable properties
// delete Object.prototype; // Error in strict mode

// 5. Prevents duplicate property names in objects
// let obj = { a: 1, a: 2 }; // Error in strict mode
let obj = { a: 1, b: 2 }; // Correct

// 6. Prevents this from being undefined in functions
function testThis() {
  console.log(this); // undefined in strict mode (instead of global object)
}

testThis(); // undefined

// 7. Prevents eval from creating variables in surrounding scope
function testEval() {
  eval('let evalVar = 42;');
  // console.log(evalVar); // Error: evalVar is not defined
}

testEval();

Strict Mode Benefits

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Common Strict Mode Errors

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Best Practices

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Migration to Strict Mode

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Tip: Always use strict mode in new projects - it helps catch errors early and makes your code more robust!