for loop

for loop

Function

for is used to repeatedly execute code, loop by integer range, infinite loop, and also used to traverse arrays, objects and strings.

Arrays are traversed by subscript and value, objects are traversed by key and value, and strings are traversed by characters. Integer and interval loops use a lazy iteration state and do not generate the complete array in advance.

Syntax

Parameters

Return value

The for statement itself returns empty. If you need to collect results, you usually create an array or object first and write it in the loop body.

Infinite loop

for {} represents an infinite loop, which needs to be exited through break, return or exit within the loop body.

Repeat N times

When no index is needed, write the count directly.

When indexing is required, use for i in count. The index starts at 0 and ends at count - 1.

Use for i in count step step_value when you need to fix the number of loops but increment the variable by a specified step size. The following will loop three times, outputting 0, 2, 4.

Interval loop

start..end is a closed interval, including left and right boundaries. It increases when the starting point is less than or equal to the end point, and decreases when the starting point is greater than the end point.

..end is equivalent to 0..end.

start.. means infinite increment from the starting point.

step

step controls how far the loop variable changes on each for iteration. A count-based loop always runs count times and increases its variable from 0 by step on each iteration. For an interval loop, the relationship between the start and end values determines the direction automatically.

Syntax

for <variable> in <count> step <step>

for <variable> in <range> step <step>

Constraints

• step must be an integer greater than 0**
• Negative step sizes are not supported (the direction is automatically determined by the interval)

Rules of conduct

1. Default step size

When step is omitted, the step size defaults to 1.

2. Number of cycles

for i in count step step_value runs count times. The values of i are 0, step_value, step_value * 2, and so on; count is not treated as the endpoint of a closed interval.

3. Automatic direction determination

• Forward iteration: when start ≤ end, the loop variable increases by step.
• Reverse iteration: when start > end, the loop variable decreases by step.

4. Interval characteristics

• The interval is a closed interval: both the starting value and the ending value will be iterated (if the step size can be reached accurately)
• Forward iteration termination condition: variable > end
• Reverse iteration termination condition: variable < end

Example

Design principles

step only defines the change range (absolute value of the step size) of each iteration. The iteration direction of the interval loop is completely determined implicitly by the size relationship between the start and the end, which avoids the redundancy of explicitly specifying negative step sizes such as -2.

Traverse the collection

When you only care about the element itself, just write a variable.

When a subscript or object key is required, write two variables: the first is the key and the second is the value. The comma between two variables can be written or omitted.

Discard binding

_ means discarding the current binding and will not create or overwrite variables named _.

Destructure the current value

When iterating over objects or arrays in an array, use parentheses after for to destructure the current value and select only the fields needed by the loop body.

The preceding form is equivalent to:

Variables inside the parentheses can be separated by spaces or commas. When the current value is an object, fields with the same names are read; when it is an array, elements are read by index. A missing field or index is assigned empty. A runtime error occurs when the current value is neither an array nor an object.

break and continue

break immediately ends the current loop; continue skips the remaining statements in the current iteration and starts the next one.

Label loop

In nested loops, you can label the loop and then use break:label or continue:label to control the specified level. The colon is used to clearly indicate that it is followed by a jump label. The label is written after the loop keyword in the format of for:a, while:a or loop:a.

Notes

• The count in for count {} must be a non-negative integer; 0 performs zero iterations.
• When count in for i in count {} is an integer, press 0..count - 1 to cycle; for i in count step step_value {} will cycle count times, and i will increase by step_value starting from 0.
• Interval loops only support integers, and the first version does not support floating point steps.
• start..end contains end.
• start.. is an infinite increment interval, which usually needs to be matched with break in the loop body.
• for value in obj still reads the object value; use for key, value in obj or for key value in obj when an object key is required.