Tuple types

Tuple types express an array where the type of certain elements is known, but need not be the same. For example, you may want to represent an array with a string at position 0 and a number at position 1:

// Declare a tuple type
var x: [string, number];
// Initialize it
x = ['hello', 10]; // OK
// Initialize it incorrectly
x = [10, 'hello']; // Error

When accessing an element with a known index, the correct type is retrieved:

console.log(x[0].substr(1)); // OK
console.log(x[1].substr(1)); // Error, 'number' does not have 'substr'

Note that in TypeScript 1.4, when accessing an element outside the set of known indices, a union type is used instead:

x[3] = 'world'; // OK
console.log(x[5].toString()); // OK, 'string' and 'number' both have toString
x[6] = true; // Error, boolean isn't number or string