Having difficulty invoking the forEach function on an Array in TypeScript

Question

Having difficulty invoking the forEach function on an Array in TypeScript

I'm currently working with a class that contains an array of objects.

export interface Filter {
  sf?: Array<{ key: string, value: string }>;
}

In my attempt to loop through and print out the value of each object inside the array using the forEach function,

f: Filter = {sf: [{key:'a', value:'b'}, {key:'c', value:'d'}]}; 

f.sf.forEach(element => {
  console.log(element.key);
});

An error message keeps popping up from the mongo db server.

[error] f.sf.forEach is not a function 0
node                           | TypeError: f.sf.forEach is not a function

Can anyone provide guidance on how to resolve this error so that the forEach method can be executed successfully? Thank you in advance.

typescript

Answer 1

Answer №1

According to the guidelines of your Filter interface, the variable sf is considered optional.

If a value is not specified, it will default to undefined unless stated otherwise. To check if a variable is undefined, there are various methods:

const myVariable = undefined
const myOtherVar = null

// Using double "==" may not always be the best approach for testing values
console.log("myVariable, ==", myVariable == undefined)
console.log("myVariable, ===", myVariable === undefined)

console.log("myOtherVar ==", myOtherVar == undefined
console.log("myOtherVar ===", myOtherVar === undefined)

It's important to note that undefined == null evaluates to true, but undefined === null results in false

To make the variable non-optional, you can define it as follows:

export interface Filter {
  sf:Array<{key:string,value:string}>;
}

Alternatively, you can express this in another way:

type SF = {
  key:string,
  value:string
}

interface Filter {
  sf: SF[];
}

Another option is to utilize optional chaining like so:

f.sf?.forEach(element => {
  console.log(element.key);
});

Answer 2

According to the guidelines of your Filter interface, the variable sf is considered optional.

If a value is not specified, it will default to undefined unless stated otherwise. To check if a variable is undefined, there are various methods:

const myVariable = undefined
const myOtherVar = null

// Using double "==" may not always be the best approach for testing values
console.log("myVariable, ==", myVariable == undefined)
console.log("myVariable, ===", myVariable === undefined)

console.log("myOtherVar ==", myOtherVar == undefined
console.log("myOtherVar ===", myOtherVar === undefined)

It's important to note that undefined == null evaluates to true, but undefined === null results in false

To make the variable non-optional, you can define it as follows:

export interface Filter {
  sf:Array<{key:string,value:string}>;
}

Alternatively, you can express this in another way:

type SF = {
  key:string,
  value:string
}

interface Filter {
  sf: SF[];
}

Another option is to utilize optional chaining like so:

f.sf?.forEach(element => {
  console.log(element.key);
});

Having difficulty invoking the forEach function on an Array in TypeScript

Answer №1

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