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Everyday TypeScript: Sets

Welcome to the Sets lesson!

This lesson is shown as static text below. However, it's designed to be used interactively. Click the button below to start!

  • Recent versions of JavaScript have added a "set" data type. Sets are collections that hold elements, in the same way that arrays do. The main difference is that a set can only contain one copy of any given element.

  • (If you'd like a refresher on sets, check out the sets lesson in our Modern JavaScript course!)

  • >
    const s = new Set([1, 2, 3]);
    s.has(1);
    Result:
  • >
    const s = new Set([1, 2, 3]);
    s.has(2);
    Result:
    truePass Icon
  • >
    const s = new Set([1, 2, 3]);
    s.has(4);
    Result:
    falsePass Icon

  • We can add elements to existing sets.

  • >
    const s = new Set([1, 2, 3]);
    s.add(4);
    s.has(4);
    Result:
    truePass Icon

  • When writing TypeScript, sets are a generic type. A Set<string> can only contain strings.

  • >
    let s: Set<string> = new Set(['a', 'b', 'c', 4]);
    s.has('a');
    Result:
    type error: Type 'Set<string | number>' is not assignable to type 'Set<string>'.
  Type 'string | number' is not assignable to type 'string'.
    Type 'number' is not assignable to type 'string'.Pass Icon

  • As usual, we can use any type as a generic type parameter, as long as it type checks. We can fix the type error above by creating a Set<string | number>.

  • >
    let s: Set<string | number> = new Set(['a', 'b', 'c', 4]);
    s.has('a');
    Result:
    truePass Icon

  • TypeScript will stop us from calling methods like has or add with the wrong type.

  • >
    let s: Set<string> = new Set(['a', 'b', 'c']);
    s.has('a');
    Result:
    truePass Icon
  • >
    let s: Set<string> = new Set(['a', 'b', 'c']);
    s.has(1);
    Result:
    type error: Argument of type 'number' is not assignable to parameter of type 'string'.Pass Icon

  • Array and Set are different types. That means that Array<string> and Set<string> are different too. We can't assign an array to a set or vice-versa.

  • >
    let strings: Set<string> = new Set(['a']);
    let strings2: Array<string> = strings;
    strings2;
    Result:
    type error: Type 'Set<string>' is missing the following properties from type 'string[]': length, pop, push, concat, and 20 more.Pass Icon

  • Here's a code problem:

    The setIsEmpty function below should decide whether the set is empty. It already has a correct function body that uses .size to perform that check. Add the missing type for the function's set parameter.

    Note that the function is generic, so it should work with any type of set: Set<number>, Set<string>, etc. You'll need to use the type parameter T as Set's type parameter. (But you won't need to change the function's body in any way!)

    function setIsEmpty<T>(set: Set<T>): boolean {
    return set.size === 0;
    }
    [
    setIsEmpty(new Set(['a'])),
    setIsEmpty(new Set([1, 2, 3])),
    setIsEmpty(new Set([])),
    ];
    Goal:
    [false, false, true]
    Yours:
    [false, false, true]Pass Icon