K. Scott Allen - What Every JavaScript Developer Should Know About ECMAScript 2015

Unfortunately , cons t is one of those keywords that has been around in some runtimes even before the keyword appeared in an official ECMAScript specification. While most runtimes can agree that a cons t should be constant, existing implementations disagree on how to behave when code attempts to assign a new value to a constant. Should the assignment cause a runtime error? Should the assignment fail silently? The official specification say s cons t should fail with a runtime error if a program tries to assign a new value to a constant, and constant declarations without an initializer should be a syntax error. const MAX_SIZE = 10; // MAX_SIZE = 12; // If executed, will result in a SyntaxError I hope it will not take long for all the latest runtimes and browsers to straighten up and behave correctly. The current situation should notstop you from usin g cons t whenever possible because the keyword does have a few benefits.

1. cons t tells other programmers about the intended behavior of a variable, and this makes a script easier to maintain.
2. cons t tells the runtime about the intended behavior of a variable, which can allow for optimizations across time and space.

In cases where we do not pass enough parameters, the author of a function might want to specify a default value instead of working wit h undefine d . Before 2015, developers applied default values to incoming parameters usin g | | expressions inside the function. In the following function, if the caller does not pass a value for th e nam e parameter, the code will assign a default of Scott. let doWork = function(name) { name = name || "Scott"; return name; }; In 2015, default values are explicit and appear in the parameter list for a function. let doWork = function(name = "Scott") { return name; }; expect(doWork()).toBe("Scott"); A function can even calculate a default using something more than just a literal expression. let doWork = function(x = Math.random()) { return x; } The default parameter syntax is a good example of how ES2015 provides a clean syntax for a common requirement.

No longer will we have to scan through a functions source code or documentation to discover the defaults. Of course, not all parameters are simple values. A common programming technique for operations involving complex configurations, like an HTTP call, is to accept a single object argument with properties to set the URI, HTTP method, timeout settings, and more. Default parameters are useful in this scenario, too, but well have to wait for later and learn about object destructuring first.