JavaScript & TypeScript

Master JavaScript and TypeScript for software engineering interviews, covering language internals, async patterns, type safety, and performance. Session 1: JS Fundamentals JavaScript's scoping model and hoisting behavior are the source of more interview questions — and production bugs — than almost any other language feature. Getting these right is foundational. var, let, and const is function-scoped and hoisted to the top of its containing function (or global scope) with an initial value of . This means you can reference a variable before its declaration line without a ReferenceError — you'll just get . and are block-scoped (bounded by ). They are also hoisted, but they are NOT initialized. Accessing them before their declaration line throws a . The period between the start of the block and the declaration line is called the Temporal Dead Zone (TDZ). additionally requires an initializer and cannot be reassigned. However, if the value is an object or array, the internal properties/elements can still be mutated. Hoisting Hoisting is JavaScript's behavior of moving declarations to the top of their scope before execution. Two things are hoisted: variable declarations (not initializations) and function declarations (fully — both name and body). Scope Chain Every function creates a new scope. When code references a variable, JavaScript looks it up in the current scope, then walks up the scope chain through enclosing scopes until it either finds the variable or reaches the global scope (where a ReferenceError is thrown if not found). Scope is lexical — it is determined at write time by where functions are defined, not where they are called. Closures A closure is a function that retains a reference to its lexical environment even after the outer function has returned. The inner function closes over the variables of its enclosing scope. Interview Insight When asked about closures, always mention the practical applications: module pattern (data privacy), factory functions, event handlers, memoization, and partial application. Then demonstrate the loop gotcha — interviewers love that one. Python Implementation