The Error object — name, message, stack

What's inside an Error

Every Error has three useful fields:

• err.name — the class name ("Error", "TypeError", "RangeError", custom).
• err.message — the first arg you passed to new.
• err.stack — a multi-line string with the call stack at throw time.

const err = new TypeError("expected number");
err.name;     // "TypeError"
err.message;  // "expected number"
err.stack;    // "TypeError: expected number\n    at <anonymous>:1:13..."

Built-in error subclasses

JavaScript ships several subclasses you should reach for instead of bare Error:

• TypeError — wrong type (e.g. called (undefined).foo).
• RangeError — number/string outside valid range.
• SyntaxError — thrown by JSON.parse on bad input.
• URIError — decodeURIComponent on bad input.

Using the right subclass lets call sites narrow with instanceof (see C7-05).