Add elements inside Array conditionally in JavaScript

When you spread into an array, you call the Symbol.iterator method on the object. && evaluates to the first falsey value (or the last truthy value, if all are truthy), so

let arr2 = ['value2', ...(condition && arr)];

results in

let arr2 = ['value2', ...(false)];

But false does not have a Symbol.iterator method.

You could use the conditional operator instead, and spread an empty array if the condition is false:

let condition = false;
let arr1 = ['value1'];
let arr2 = ['value2', ...(condition ? arr1 : [])];
console.log(arr2);

(This works because the empty array does have the Symbol.iterator method)

Object spread is completely different: it copies own enumerable properties from a provided object onto a new object. false does not have any own enumerable properties, so nothing gets copied.

false is not spreadable.

You need a spreadable object (the one where Symbol.iterator is implemented) which returns nothing, if spreaded.

You could use an empty array as default value. This works even if arr is falsy.

let condition = false;
let arr1 = ['value1'];
let arr2 = ['value2', ...(condition && arr || [])];

console.log(arr2);

This is a specification difference between the spread syntax for object literals and for array literals.

MDN briefly mentions it here -- I highlight:

Spread syntax (other than in the case of spread properties) can be applied only to iterable objects

The difference comes from the EcmaScript 2018 specification:

• Concerning object spread syntax, see 12.2.6.8 Runtime Semantics: PropertyDefinitionEvaluation:

  • It calls CopyDataProperties(object, fromValue, excludedNames) where the fromValue is wrapped to an object with ToObject, and therefore becomes iterable, even if fromValue is a primitive value like false. Therefore {...false} is valid EcmaScript.

• Concerning array spread syntax, see 12.2.5.2 Runtime Semantics: ArrayAccumulation:

  • It merely calls GetValue(spreadRef) which does not do the above mentioned wrapping. And so the subsequent call to GetIterator will trigger an error on a primitive value, as it is not iterable. Therefore [...false] is invalid EcmaScript.