When your program contains more than one statement, the statements are executed as if they are a story, from top to bottom. Like any good programming book, Eloquent JavaScript begins with fundamentals--variables, control structures, functions, and data structures--then moves on to complex topics like object-oriented programming and regular expressions. A call to such a function can be substituted by its return value without changing the meaning of the code. For the clever solution, build up a string containing the word or words to output and print either this word or the number if there is no word, potentially by making good use of the || operator. With this modification, if you enter “parrot”, no output is shown. If that does not work, search through the menus for an item named Developer Tools or similar. The counter could also have started at 1 and checked for <= 10, but for reasons that will become apparent in Chapter 4, it is a good idea to get used to counting from 0. With proper indentation, the visual shape of a program corresponds to the shape of the blocks inside it. The parentheses after a for keyword must contain two semicolons. // It's a green hollow where a river sings. Functions that donât have a return statement at all, such as makeNoise, similarly return undefined. Eloquent JavaScript. Remember the trick of using the remainder (%) operator for checking whether a number is divisible by another number (has a remainder of zero). Each block creates a new scope. Your function will need a loop that looks at every character in the string. Worrying about efficiency can be a distraction. If that fails, you will have to close the tab that you’re working in, or on some browsers close your whole browser, to recover. These are the sources used to build the third edition of Eloquent JavaScript (https://eloquentjavascript.net). It always contains bindings that are part of the language standard, and most of the time, it also has bindings that provide ways to interact with the surrounding system. When running the examples (or your own code) on the pages of this book, console.log output will be shown after the example, instead of in the browser’s JavaScript console. Therefore, we usually have to introduce new concepts to avoid repeating ourselves too much. READ PAPER. Write a program that uses console.log to print all the numbers from 1 to 100, with two exceptions. Thinking about programs like this takes some practice. You might even start writing code that uses the function before you actually define the function itself. Recursion. // Madly catching white tatters in the grass. Using the remainder (%) operator is an easy way to test whether a number is divisible by another number. A newline character is written "\n". You could also have omitted them in this case, since they hold only a single statement, but to avoid having to think about whether they are needed, most JavaScript programmers use them in every wrapped statement like this. How difficult it is to find a good name for a function is a good indication of how clear a concept it is that youâre trying to wrap. Letâs take a closer look at it. This much anticipated and thoroughly revised third edition of Eloquent JavaScript dives deep into the JavaScript language to show you how to write beautiful, effective code. The function body of a function created this way must always be wrapped in braces, even when it consists of only a single statement. A single let statement may define multiple bindings. The function calls itself multiple times with ever smaller exponents to achieve the repeated multiplication. To better understand how this function produces the effect weâre looking for, letâs look at all the calls to find that are made when searching for a solution for the number 13. Creating values like this is the main substance of any JavaScript program. It can run an index from zero to one below its length (< string.length). This program is exactly equivalent to the earlier even-number-printing example. It is perfectly okay for a function to call itself, as long as it doesnât do it so often that it overflows the stack. Similar shortcuts work for many other operators, such as result *= 2 to double result or counter -= 1 to count downward. Don’t worry about memorizing this list. These spaces are not required—the computer will accept the program just fine without them. With a slight change, we can turn the previous example into a way to create functions that multiply by an arbitrary amount. Itâs yet another factor that complicates program design, and when youâre doing something thatâs already difficult, that extra thing to worry about can be paralyzing. A lot of functions are useful because of the side effects they produce. So, these two definitions of square do the same thing: When an arrow function has no parameters at all, its parameter list is just an empty set of parentheses. In the simple case, we want some code to be executed if, and only if, a certain condition holds. Written by Marijn Haverbeke. This asks for a function of two argumentsâthe number of cows and the number of chickens. These are two different features, neither of which depend on the either. Words with a special meaning, such as let, are keywords, and they may not be used as binding names. Usually you’ll directly use the name of the binding that holds the function. If you have more than two paths to choose from, you can “chain” multiple if/else pairs together. Hereâs a first attempt: It works! In one case, console.log has to return to the greet function when it is done. Functions can be roughly divided into those that are called for their side effects and those that are called for their return value. Digits can be part of binding names—catch22 is a valid name, for example—but the name must not start with a digit. This is called conditional The loop tests whether the second binding has reached 10 yet and, if not, updates both bindings. There the || operator causes the call that explores (1 * 3) to happen. Eloquent JavaScript. This is because console.log isn’t a simple binding. The first character has position 0, which causes the last one to be found at position string.length - 1. This situation is a good demonstration of the fact that local bindings are created anew for every call, and different calls canât trample on one anotherâs local bindings. The full list of keywords and reserved words is rather long. We could write anything, from a terribly simple function that can only pad a number to be three characters wide to a complicated generalized number-formatting system that handles fractional numbers, negative numbers, alignment of decimal dots, padding with different characters, and so on. Almost any program can be made faster by making it bigger and more convoluted. We’ve seen that % (the remainder operator) can be used to test whether a number is even or odd by using % 2 to see whether it’s divisible by two. Thus, your solution can closely follow the number-printing program. In the following example, makeNoise does not list any parameter names, whereas power lists two: Some functions produce a value, such as power and square, and some donât, such as makeNoise, whose only result is a side effect. You often won’t just have code that executes when a condition holds true, but also code that handles the other case. 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