Unable to call forEach function from an Array in typescript

Question

I have a class with an array of object.

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

I try to use forEach function to loop and log each object value inside the array.

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

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

However error appears inside the mongo db server.

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

how can I fix this error so the forEach works? thank you.

Could it be that f.sf is undefined as permitted by the Filter interface definition? — BryceCicada
– BryceCicada, Commented Dec 15, 2022 at 9:28

Harrison · Accepted Answer · 2022-12-15 11:07:38Z

1

As stated by your Filter interface, sf is optional.

Optional values will default to undefined (unless otherwise stated). You can check if a variable is not defined in many ways:

const myVariable = undefined
const myOtherVar = null

// double "=", not always a great way to test the value
console.log("myVariable, ==", myVariable == undefined)
console.log("myVariable, ===", myVariable === undefined)

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

Note how undefined == null is true but undefined === null is false

You either need to make it non-optional like so:

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

Another way of writing this is:

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

interface Filter {
  sf: SF[];
}

Or use optional chaining like this:

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

edited Dec 15, 2022 at 11:07

answered Dec 15, 2022 at 9:29

Harrison

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5 Comments

AkiZukiLenn Over a year ago

After trying the optional chaining, it returns error TypeError: _a.forEach is not a function...

AkiZukiLenn Over a year ago

Hi I also added a if(f.sf!=null) statement outside the forEach function, since my post's edit function is bug, I want to tell you this addition info here. Hope it is useful.

AkiZukiLenn Over a year ago

I also checked the object with function Array.isArray(f.sf), seems like it is false...

Harrison Over a year ago

@AkiZukiLenn Optional objects will default to undefined, not necessarily null. null and undefined can be tricky, because: undefined != null // false BUT undefined !== null // true. Also if you got _a.forEach is not a function then you have the same optional issue for the variable a (Provided a is an optional array too)

AkiZukiLenn Over a year ago

Found the problem, its outside of this topic scope. Turns out angular form.value do not return array, its an object. Have to map it back into an array.

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