iterate-objects.md

Item 60: Know How to Iterate Over Objects

Things to Remember

• Be aware that any objects your function receives as parameters might have additional keys.
• Use Object.entries to iterate over the keys and values of any object.
• Use a ++for-in++ loop with an explicit type assertion to iterate objects when you know exactly what the keys will be.
• Consider Map as an alternative to objects since it's easier to iterate over.

//// Check that the "Surprisingly" aside continues to work: // verifier:reset // verifier:prepend-subset-of-id-to-following:abc:1-5 [source,ts]

function foo(abc: ABC) { let k: keyof ABC; for (k in abc) { // ^? let k: keyof ABC (equivalent to "a" | "b" | "c") const v = abc[k]; // ^? const v: string | number } }

////

Code Samples

const obj = {
  one: 'uno',
  two: 'dos',
  three: 'tres',
};
for (const k in obj) {
  const v = obj[k];
  //        ~~~~~~ Element implicitly has an 'any' type
  //               because type ... has no index signature
}

💻 playground

---

const obj = { one: 'uno', two: 'dos', three: 'tres' };
//    ^? const obj: {
//         one: string;
//         two: string;
//         three: string;
//       }
for (const k in obj) {
  //       ^? const k: string
  // ...
}

💻 playground

---

for (const kStr in obj) {
  const k = kStr as keyof typeof obj;
  //    ^? const k: "one" | "two" | "three"
  const v = obj[k];  // OK
}

💻 playground

---

interface ABC {
  a: string;
  b: string;
  c: number;
}

function foo(abc: ABC) {
  for (const k in abc) {
    //       ^? const k: string
    const v = abc[k];
    //        ~~~~~~ Element implicitly has an 'any' type
    //               because type 'ABC' has no index signature
  }
}

💻 playground

---

const x = {a: 'a', b: 'b', c: 2, d: new Date()};
foo(x);  // OK

💻 playground

---

function foo(abc: ABC) {
  for (const kStr in abc) {
    let k = kStr as keyof ABC;
    //  ^? let k: keyof ABC (equivalent to "a" | "b" | "c")
    const v = abc[k];
    //    ^? const v: string | number
  }
}

💻 playground

---

function foo(abc: ABC) {
  for (const [k, v] of Object.entries(abc)) {
    //        ^? const k: string
    console.log(v);
    //          ^? const v: any
  }
}

💻 playground

---

function foo(abc: ABC) {
  const keys = ['a', 'b', 'c'] as const;
  for (const k of keys) {
    //       ^? const k: "a" | "b" | "c"
    const v = abc[k];
    //    ^? const v: string | number
  }
}

💻 playground

---

const m = new Map([
  //  ^? const m: Map<string, string>
  ['one', 'uno'],
  ['two', 'dos'],
  ['three', 'tres'],
]);
for (const [k, v] of m.entries()) {
  //        ^? const k: string
  console.log(v);
  //          ^? const v: string
}

💻 playground