Here's the code with error:

    let createArray = function<T>(length: number, value: T): T[] {
        let result: T[] = [];
        for (let i = 0; i < length; i  ) {
            if (typeof value === 'string') {
                result[i] = value   '_'   i;  // Type 'string' is not assignable to type 'T'. 'T' could be instantiated with an arbitrary type which could be unrelated to 'string'.
            } else {
                result[i] = value;
            }
            
        }
        return result;
    }
    
    createArray(3, 'x');

I have no idea why Typescript can not recognize the Generics T as string in this case.

How can I fix it?

CodePudding user response：

Use Overloading:

function createArray(length: number, value: string): string[];

function createArray<T>(length: number, value: T): T[];

function createArray<T>(length: number, value: T|string): (T|string)[] {
    let result = [];
    for (let i = 0; i < length; i  ) {
        if (typeof value === 'string') {
            result[i] = value   '_'   i;  // Type 'string' is not assignable to type 'T'. 'T' could be instantiated with an arbitrary type which could be unrelated to 'string'.
        } else {
            result[i] = value;
        }
        
    }
    return result;
}

console.log(createArray(3, 'x'));           // ["x_0", "x_1", "x_2"] 

console.log(createArray(3, 23));            // [23, 23, 23]

console.log(createArray(3, {hi: "hello"})); // [{
                                            //   "hi": "hello"
                                            // }, {
                                            //   "hi": "hello"
                                            // }, {
                                            //   "hi": "hello"
                                            // }] 

See demo.