If your document contains an array field, you can use arrayUnion() and arrayRemove() to add and remove elements. arrayUnion() adds elements to an array but only elements not already present. arrayRemove() removes all instances of each given element.

The Array Update operation is only available for Array fields and will only work if the Field has either been declared during a Add operation or has been updated with a Update operation. If the Field doesn’t exist, the Array Update operation will fail.

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Add Elements to an Array

To add elements to an array field, use the arrayUnion() method:

db.collection('NodeJSDb1').doc('de79272c-2ed3-46d9-ae93-ff195364466e').update({
    array: db.arrayUnion(["a"])
}).then((response) => {
    console.log(response);
}).catch((error) => {
    console.log(error);
});

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Remove Elements from an Array

To remove elements from an array field, use the arrayRemove() method:

db.collection('NodeJSDb1').doc('de79272c-2ed3-46d9-ae93-ff195364466e').update({
    array: db.arrayRemove(["a"])
}).then((response) => {
    console.log(response);
}).catch((error) => {
    console.log(error);
});