JavaScript Essentials
Variables (let/const/var), types, closures, this, arrow functions, destructuring, spread/rest
1What is the main difference between let and var in JavaScript?
What is the main difference between let and var in JavaScript?
Answer
let has block scope while var has function scope. This means a variable declared with let is only accessible within the block where it's defined (between curly braces), whereas var is accessible throughout the entire function. This difference is crucial to avoid bugs related to hoisting and variables leaking outside their intended context.
2What happens when accessing a let variable before its declaration?
What happens when accessing a let variable before its declaration?
Answer
A ReferenceError is thrown because the variable is in the Temporal Dead Zone (TDZ). Unlike var which is hoisted with undefined value, let variables are hoisted but remain inaccessible until their declaration. This temporal dead zone helps detect programming errors where a variable is used before being defined.
3What is the main characteristic of const in JavaScript?
What is the main characteristic of const in JavaScript?
Answer
const prevents reference reassignment but not content mutation. For primitive types, this means the value cannot change. For objects and arrays, the reference stays the same but properties or elements can be modified. Using const by default is good practice as it expresses the intent not to reassign the variable.
What are the primitive types in JavaScript?
What is the result of typeof null in JavaScript?
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