What is TypeScript?

TypeScript is the industry's answer to a famous weakness of JavaScript: the web's language is permissive to the point of danger — a variable that held a number becomes text, a function receives data it never expected, and the error detonates only at runtime, in a customer's browser, on a Friday evening. TypeScript — created by Microsoft, open source — layers a type system over JavaScript: developers declare what kind of data flows where (“this function takes an order with products and returns an invoice”), and the compiler verifies everything before execution — whole categories of mistakes die in the editor rather than in production. Its relationship to JavaScript is that of a superset: TypeScript compiles down to ordinary JavaScript, runs wherever JavaScript runs, and can be adopted gradually. The difference shows at scale: on tiny projects it's a matter of taste; on business applications with tens of thousands of lines and rotating teams, types become living documentation (signatures state what each piece takes and returns, so newcomers write with informed autocompletion instead of archaeology) and a seatbelt for change: alter a data structure and the compiler lists every place needing an update — precisely the category of slips that in plain JavaScript surfaces through complaints. Hence its current status: the major frameworks treat it as the default, and the talent market considers it baseline. For a buyer, three lines suffice: “we build in TypeScript” signals engineering normality; an inherited plain-JavaScript application is no emergency, since gradual migration happens naturally during maintenance; and the quality question worth asking — “strict TypeScript, or in name only?” — since half-disabled type checking ticks the box without delivering the net.