how to use the isset function from the underscore library in typescript

To use the isset function from underscore in Typescript, you first need to install the Underscore typings. You can install them using NPM by running the following command:

index.ts
npm install @types/underscore
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Once you have the typings installed, you can import the underscore library and use the _.isUndefined() method to check if a variable is defined or not. Here's an example:

index.ts
import * as _ from 'underscore';

let myVar: any;

if (_.isUndefined(myVar)) {
    console.log('myVar is undefined')
} else {
    console.log('myVar is defined')
}
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In this example, we import the entire Underscore library as _, declare a variable called myVar without initializing it, and then use the _.isUndefined() method to check if it's defined or not. If it's undefined, we log a message saying so, otherwise, we log a message saying it's defined.

Alternatively, you could also define your own function to check if a variable is defined, like this:

index.ts
function isset(variable: any): boolean {
    return typeof variable !== 'undefined';
}

let myVar: any;

if (!isset(myVar)) {
    console.log('myVar is undefined')
} else {
    console.log('myVar is defined')
}
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In this case, we define a function called isset, which checks if a variable is defined or not by comparing its type to 'undefined'. We then use this function to check the myVar variable, and log a message saying if it's defined or not.