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JavaScript Asynchronous Programming with JavaScript Understanding Promises Promises Review

Wendy Cortes
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Wendy Cortes
Full Stack JavaScript Techdegree Graduate 16,375 Points

For the example of mathPromise, why .then(addFive) and not .then(addfive(value))

for this part:

const value = 5; mathPromise .then(addFive) .then(double) .then(finalValue) .catch( err => console.log(err) )

How are we calling the functions inside the methods without including the parameter value?

This has been the hardest part for me when understanding promises. The callback function, parameter, argument, etc.

For instance, just looking at this part mathPromise.then(addFive)

The return or resolved (value) from mathPromise is implied as passed as an argument. At least that's the way I think of it.

mathPromise.then(addFive) === mathPromise.then(value => addFive(value))

In the left side of the equation, even though value isn't explicitly written out, it's implied.

Someone please feel free to correct me if I'm mistaken.

1 Answer

Steven Parker
Steven Parker
229,005 Points

Just naming the function doesn't call it, it passes it to the handler which will call it later when the "then" condition is satisfied. At that time, it will be passed the value which the "then" is waiting for.

It wouldn't make sense to call it right now since the value needed to pass to it isn't available yet.