Error: expecting an array, a promise or a thenable

When this error occurs, the function throwing the error expects a return value that is Promisified, but is given something different. There are two main reasons why this may occur.

1. Working with collections (like arrays) but pass a single, non-collection element instead

Example:

function returnThree(){ return 3;}

Promise.resolve(5).map(returnThree).then(function(val){
     console.log("Hello Value!",val); 
});

The map operation is expecting an array here (or a promise on one) and instead gets the number 5.

function returnThree(){ return 3;}

Promise.resolve([5]).map(returnThree).then(function(val){
     console.log("Hello Value!",val); 
});

map is given an array with a single element (see [5] instead of 5), so this statement will work (but is bad practice).

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2.return is forgotten in a 'fat' arrow / anonymous function call =>:

When debugging or performing a one-time operation on a variable before passing it to a function, a return variable is forgotten.

Example:

function nextFunction(something){ return Promise.resolve(something*3); }

myFunction()
    .then(result => nextFunction(result)); // We are implicitly returning a Promise

Debugging, we want to see the value of result, so we add a console.log() line:

function nextFunction(something){ return Promise.resolve(something*3); }

myFunction().then(result => {
    console.log("Debug:", result);
    nextFunction(result)); // The chain is broken! We don't return anything to the .then() call
});

As this is an anonymous function call, we need to return something, which is not currently happening.

To fix, simply remember to add return in front of your promise-complying function:

function nextFunction(something){ return Promise.resolve(something*3); }

myFunction().then(result => {
    console.log("Debug:", result);
    return nextFunction(result)); // The anonymous function returns the function which returns the promise .then() needs
});