JavaScript 函数参数
A JavaScript function does not perform any checking on parameter values (arguments).
Function Parameters and Arguments
Earlier in this tutorial, you learned that functions can have 参数:
功能 函数名称(parameter1, parameter2, parameter3) {
// 要执行的代码
}
Function 参数 are the names listed in the function definition.
Function arguments are the real values passed to (and received by) the function.
Parameter Rules
JavaScript function definitions do not specify data types for parameters.
JavaScript functions do not perform type checking on the passed arguments.
JavaScript functions do not check the number of arguments received.
Default Parameters
If a function is called with missing arguments (less than declared), the missing values are set to undefined.
Sometimes this is acceptable, but sometimes it is better to assign a default value to the parameter:
Default Parameter Values
ES6 allows function parameters to have default values.
例子
If y is not passed or undefined, then y = 10.
function myFunction(x, y = 10) {
return x + y;
}
myFunction(5);
亲自尝试 »
Function Rest Parameter
The rest parameter (...) allows a function to treat an indefinite number of arguments as an array:
例子
function sum(...args) {
let sum = 0;
for (let arg of args) sum += arg;
return sum;
}
let x = sum(4, 9, 16, 25, 29, 100, 66, 77);
亲自尝试 »
The Arguments Object
JavaScript functions have a built-in object called the arguments object.
The argument object contains an array of the arguments used when the function was called (invoked).
This way you can simply use a function to find (for instance) the highest value in a list of numbers:
例子
x = findMax(1, 123, 500, 115, 44, 88);
function findMax() {
let max = -Infinity;
for (let i = 0; i < arguments.length; i++) {
if (arguments[i] > max) {
max = arguments[i];
}
}
return max;
}
亲自尝试 »
Or create a function to sum all input values:
例子
x = sumAll(1, 123, 500, 115, 44, 88);
function sumAll() {
let sum = 0;
for (let i = 0; i < arguments.length; i++) {
sum += arguments[i];
}
return sum;
}
亲自尝试 »
If a function is called with too many arguments (more than declared), these arguments can be reached using the arguments object.
Arguments are Passed by Value
The parameters, in a function call, are the function's arguments.
JavaScript arguments are passed by value: The function only gets to know the values, not the argument's locations.
If a function changes an argument's value, it does not change the parameter's original value.
Changes to arguments are not visible (reflected) outside the function.
Objects are Passed by Reference
In JavaScript, object references are values.
Because of this, objects will behave like they are passed by reference:
If a function changes an object property, it changes the original value.
Changes to object properties are visible (reflected) outside the function.
