Tuple
Tuple types allow you to express an array where the type of a fixed number of elements is known, but need not be the same. For example, you may want to represent a value as a pair of a string and a number:
// Declare a tuple type let 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'
When accessing an element outside the set of known indices, a union type is used instead:
x[3] = "world"; // OK, 'string' can be assigned to 'string | number' console.log(x[5].toString()); // OK, 'string' and 'number' both have 'toString' x[6] = true; // Error, 'boolean' isn't 'string | number'
Union types are an advanced topic that we’ll cover in a later chapter.
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