how to use the isdate function from the lodash library in typescript

• typescript
• isdate
• type declaration
• type assertion

To use the isDate function from the lodash library in TypeScript, you first need to install the @types/lodash package to get the corresponding type declarations:

index.ts
npm install --save-dev @types/lodash
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Then, you can import the isDate function from lodash and use it in your TypeScript code. However, because the isDate function has a union type of Date | null | undefined, TypeScript does not automatically infer that the value is a Date after passing a type guard like if (lodash.isDate(value)).

To address this issue, you can use a type assertion to tell TypeScript that the value is definitely a Date. Here's an example:

index.ts
import * as lodash from 'lodash';

function logDateOrUndefined(value: Date | null | undefined) {
  if (lodash.isDate(value)) {
    console.log(value.toISOString());
    const date: Date = value; // type assertion
    console.log(date.getFullYear());
  } else {
    console.log(value);
  }
}

logDateOrUndefined(undefined); // prints "undefined"
logDateOrUndefined(new Date()); // prints an ISO string and a year
logDateOrUndefined(null); // prints "null"
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