for (variable in object) statement

variable
Receives a string property name on each iteration. May be either a declaration with const, let, or var, or an assignment target (e.g., a previously declared variable, an object property, or a destructuring pattern). Variables declared with var are not local to the loop, i.e., they are in the same scope the for...in loop is in.

object
Object whose non-symbol enumerable properties are iterated over.

statement
A statement to be executed on every iteration. May reference variable. You can use a block statement to execute multiple statements.

for (initialization; condition; afterthought)
statement

initialization Optional
An expression (including assignment expressions) or variable declaration evaluated once before the loop begins. Typically used to initialize a counter variable. This expression may optionally declare new variables with var or let keywords. Variables declared with var are not local to the loop, i.e., they are in the same scope the for loop is in. Variables declared with let are local to the statement.

The result of this expression is discarded.

condition Optional
An expression to be evaluated before each loop iteration. If this expression evaluates to true, statement is executed. If the expression evaluates to false, execution exits the loop and goes to the first statement after the for construct. This conditional test is optional. If omitted, the condition always evaluates to true. afterthought Optional An expression to be evaluated at the end of each loop iteration. This occurs before the next evaluation of condition. Generally used to update or increment the counter variable.

statement A statement that is executed as long as the condition evaluates to true. You can use a block statement to execute multiple statements. To execute no statement within the loop, use an empty statement (;).