javascript - Error while trying to destructure an array of possible length 2 - Stack Overflow

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I am trying to destructure an array of length 2, but I get a typescript error:

[ts] Tuple type '[string]' with length '1' cannot be assigned
to tuple with length '2'.

---

    let output = {status: false};
    if(execute.permission) {
        let message: [string] = execute.params;
        if(message.length >= 2) {
            // Destructuring follows
            [output['position'], output['message']] = message;
        }
    }

How do I tell typescript, that the array could possiblly be of length 2?

I am trying to destructure an array of length 2, but I get a typescript error:

[ts] Tuple type '[string]' with length '1' cannot be assigned
to tuple with length '2'.

---

    let output = {status: false};
    if(execute.permission) {
        let message: [string] = execute.params;
        if(message.length >= 2) {
            // Destructuring follows
            [output['position'], output['message']] = message;
        }
    }

How do I tell typescript, that the array could possiblly be of length 2?

• javascript
• typescript
• ecmascript-6
• destructuring

Share Improve this question Follow asked Mar 31, 2017 at 1:13 Suhail GuptaSuhail Gupta 23.3k6666 gold badges211211 silver badges344344 bronze badges 3

• You can use: let message: [string, string] = execute.params; – Diullei Commented Mar 31, 2017 at 1:23
• @Diullei they can but they shouldn't – Aluan Haddad Commented Mar 31, 2017 at 2:35
• 1 You used a [string] instead of string[]. The former creates a tuple, and the latter creates an array. See typescriptlang.org/docs/handbook/2/everyday-types.html – Janac Meena Commented Jun 27, 2021 at 20:21

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3 Answers 3

Sorted by: Reset to default 11

You've not declared message as an array; you've declared it as a tuple ([string]) and tuples have a fixed number of elements. (See the Tuple section in the Basic Types documentation.)

You could declare it as a tuple that has two string elements ([string, string]), but given that you are testing message.length >= 2 it seems likely you intended to declare it as a string array (string[]):

let output = {status: false};
if(execute.permission) {
    let message: string[] = execute.params;
    if(message.length >= 2) {
        // Destructuring follows
        [output['position'], output['message']] = message;
    }
}

Use string[] (an array of any length) instead of [string] (a "tuple" limited to length 1) as your type.

Tuples have a specific length and make it easier to represent multiple types assigned to specific index positions, like [string, number]. Homogenous (single-type) tuples are still useful in some scenarios (such as representing pairs in a map), but are not as common.

Arrays, on the other hand, are lists of variable length but are designed to hold only references of a single type (even if that type is a union type or any). If one index of any array can hold a certain value, every index can hold that same kind of value.

---

TypeScript Code (Playground Link)

let execute = { permission: true, params: ['a', 'b']}


let output = { status: false };
    if(execute.permission) {
        let message: string[] = execute.params;
        if(message.length >= 2) {
            // Destructuring follows
            [output['position'], output['message']] = message;
        }
}

console.log(output)

Try to change type of message variable. Array of strings suits better than array with single string

let message: string[] = execute.params;

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