Javascript +=（加等号）如何工作？
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How does += (plus equal) work?
提问by muudless
I'm a bit confused with the += sign. How does it work?
我对 += 符号有点困惑。它是如何工作的？
1 += 2// equals ?and this
var data = [1,2,3,4,5]; var sum = 0; data.forEach(function(value) { sum += value; }); sum = ?
1 += 2// 等于 ？和这个
var data = [1,2,3,4,5]; var sum = 0; data.forEach(function(value) { sum += value; }); sum = ?
回答by lawnsea
1 += 2is a syntax error (left-side must be a variable).
1 += 2是语法错误（左侧必须是变量）。
x += yis shorthand for x = x + y.
x += y是 的简写x = x + y。
回答by Paul
1) 1 += 2 // equals ?
1) 1 += 2 // 等于 ?
That is syntactically invalid. The left side must be a variable. For example.
这在语法上是无效的。左边必须是一个变量。例如。
var mynum = 1;
mynum += 2;
// now mynum is 3.
mynum += 2;is just a short form for mynum = mynum + 2;
mynum += 2;只是一个简短的形式 mynum = mynum + 2;
2)
2)
var data = [1,2,3,4,5];
var sum = 0;
data.forEach(function(value) {
sum += value;
});
Sum is now 15. Unrolling the forEach we have:
总和现在是 15。展开我们拥有的 forEach：
var sum = 0;
sum += 1; // sum is 1
sum += 2; // sum is 3
sum += 3; // sum is 6
sum += 4; // sum is 10
sum += 5; // sum is 15
回答by Tudor Constantin
That is just a short form for:
这只是一个简短的形式：
sum = sum + value;
回答by Paul
+=in JavaScript (as well as in many other languages) adds the right hand side to the variable on the left hand side, storing the result in that variable. Your example of 1 +=2therefore does not make sense. Here is an example:
+=在 JavaScript（以及许多其他语言）中，将右侧添加到左侧的变量中，将结果存储在该变量中。1 +=2因此，您的示例没有意义。下面是一个例子：
var x = 5;
x += 4; // x now equals 9, same as writing x = x + 4;
x -= 3; // x now equals 6, same as writing x = x - 3;
x *= 2; // x now equals 12, same as writing x = x * 2;
x /= 3; // x now equals 4, same as writing x = x / 3;
In your specific example the loop is summing the numbers in the array data.
在您的具体示例中，循环正在对数组中的数字求和data。
回答by cillierscharl
+=operator is used to concatenate strings or add numbers.
+=运算符用于连接字符串或添加数字。
It will increment your sum variable with the amount next to it.
它将使用旁边的金额增加您的 sum 变量。
var sum = 0;
var valueAdded = 5;
sum += valueAdded;
sum = 5
总和 = 5
回答by Paolo
You have to know that:
你必须知道：
Assignment operators syntax is:
variable = expression;For this reason
1 += 2->1 = 1 + 2is not a valid syntax as the left operand isn't a variable. The error in this case isReferenceError: invalid assignment left-hand side.x += yis the short form forx = x + y, wherexis the variable andx + ythe expression.The result of the sum is 15.
赋值运算符语法是：
variable = expression;由于这个原因
1 += 2->1 = 1 + 2不是有效的语法，因为左操作数不是变量。在这种情况下的错误是ReferenceError: invalid assignment left-hand side。x += y是 的缩写x = x + y，其中x是变量和x + y表达式。总和的结果是15。
sum = 0;
sum = sum + 1; // 1
sum = sum + 2; // 3
sum = sum + 3; // 6
sum = sum + 4; // 10
sum = sum + 5; // 15
Other assignment operatorshortcuts works the same way (relatively to the standard operations they refer to). .
其他赋值运算符快捷方式的工作方式相同（相对于它们所指的标准操作）。.
回答by Sam Dutton
...and don't forget what happens when you mix types:
...不要忘记混合类型时会发生什么：
x = 127;
x += " hours "
// x is now a string: "127 hours "
x += 1 === 0;
// x is still a string: "127 hours false"
回答by ManiShankar
that's just a shorthand notation in most languages.which means that
这只是大多数语言的速记符号。这意味着
x=x+1;
x=x+1;
we can do the same operation for x-=1,x*=1,x/=1; which means
我们可以对x-=1,x*=1,x/=1做同样的操作；意思是
> x=x-1; x=x*1; x=x/1;
> x=x-1; x=x*1; x=x/1;
回答by OluO
As everyone said above
就像上面大家说的
var str = "foo"
str += " bar"
console.log(str) //will now give you "foo bar"
Check this out as well https://www.sitepoint.com/shorthand-javascript-techniques/
也检查一下https://www.sitepoint.com/shorthand-javascript-techniques/
回答by Timothy Jones
a += bis shorthand for a = a +bwhich means:
a += b是简写a = a +b，意思是：
1) 1 += 2// won't compile
1) 1 += 2// 不会编译
2) 15
2) 15
