WEBVTT

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Now that we know how to specify grammars for well-balanced expressions

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and arithmetic and well-balanced webpage tags in HTML,

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we're going to return our attention to JavaScript.

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JavaScript is actually very similar to Python.

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Just like I showed you a formal grammar for HTML,

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we're going to work our way up to seeing a formal grammar for JavaScript.

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But before we get there, I just want to make sure

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that we really understand how JavaScript programs are interpreted.

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I'm going to show you a few more in Python and in JavaScript for comparison.

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Over here on the left, I have a Python function

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that computes the absolute value of its integer argument.

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If you give me a negative number, like -5, I will return positive 5.

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If you give me a positive number, like +9 million, I will return 9 million.

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The return value of this function is always either a zero a positive number.

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Now I'm going to write the same thing in JavaScript

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to provide for a comparison.

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Everything I've drawn in blue is a special keyword or punctuation mark used by the language.

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For example, to define a function in Python we use "def."

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In JavaScript, we write out the word function, but then it's still our choice what to call it--

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I'm called it "absval" in both cases--

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and how many parameters it should receive and what they're names are.

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In both cases, we have one parameter named x.

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In Python we use colons and tabbing to tell what the body of a function is,

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what the then branch for an if is, what the else branch for an if is.

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In JavaScript, we use curly braces and closing curly braces

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to denote this sort of lexical or syntactic scope.

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This is sort of curly brace 1, and it matches up with closed curly brace 1 over here,

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2 matches up with 2, and 3 matches up with 3.

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But in general the logical structure, the flow or the meaning, is the same.

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In both cases we check to see if x is less than 0, and we return 0 minus x in that case,

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or just x alone in the other case.

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One of the most important operations in any language is printing out information,

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displaying it to the screen so that we can see the result of computation

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or just to help us debug.

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In Python we use the print procedure.

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We pass it a bunch of strings.

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Here I'm adding together the strings "hello" and exclamation mart

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to make a very enthusiastic greet--"hello!"

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Over here on the right, I'm showing the same thing in JavaScript.

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The equivalent of "print" is "document.write" or perhaps just "write."

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In this class, we'll almost always abbreviate it down to just "write" to save space.

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If you're familiar with object-oriented programming, which is not required for this class,

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you might guess what the dot is about.

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We might talk more about that later.

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One of the key differences, though, is that all of our JavaScript functions have to have

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these open and close parentheses like a mathematical function

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has parentheses around its argument.