Intro #

We can think of a JavaScript program as a collection of functions.

If we can figure out how to write a function, we just have to do that several times to write a whole program.

Object Definitions #

It’s frequently useful to name a type of object. For example, we might say:

// A Car has these fields:
//  - tankSize: Number, in gallons
//  - mileage: Number, in miles/gallon

Once we’ve done that, we can talk about Car in our signatures.

How to Write a Function #

Here are five steps:

• Signature - What type of data are the function inputs and output?
• Names - What’s the name of the function and its arguments?
• Examples - What are some inputs and corresponding outputs?
• Standard pattern - Based on the types, what’s the most likely thing we’re doing in the function?
• Code - What does the function need to do?

Standard Pattern #

• What are our function arguments? We should use those.
• For each one, we should do the standard thing with it based in its type.
• What are we returning? How do we build it?

Pattern: Simple Data #

With simple data, there’s no complex pattern.

• For arguments, we should use them.
• For a return value, we should calculate and return it.

Pattern: Object #

Each object should have an object definition.

An object has several fields.

• For arguments, we should access the fields, either with dot notation or by unpacking the object into local varaibles.
• If a field has a complex type, we should either follow its pattern or call a helper function to process it.
• To return an object, we first need to construct it by assigning appropriate values to its fields.

Pattern: List #

For each list, we should specify what type of thing it contains.

• For arguments, we should loop through the items in the list (e.g. for (let xx of xs) { … } ).
• For a return value, we should start with an empty list and then add values at the end with the .push method.