keyword iota is used on const enum, it defines the auto-incremental counter starting from 0
for ex

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const (
	Sunday    = iota // starting from 0
	Monday           // 1
	Tuesday          // 2
	Wednesday        // 3
	Thursday         // 4
	Friday           // 5
	Saturday         // 6
)

sample 1

another example

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const (
	_        = iota             // iota = 0
	KB int64 = 1 << (10 * iota) // iota = 1
	MB                          // MB = 1 << (10*iota) but iota = 2
	GB                          // same as GB, but iota is 3
	TB                          // iota = 4
)

run it, you would see result as this :

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1024
1048576
1073741824
1099511627776

code

in a single const enum, you could provide multiple iota, each of them would increase independtly

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const (
	AA, BB = iota, iota << 10 // 0, 0 << 10
	CC, DD                    // 1, 1 << 10
)

result : 

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0
0
1
1024

if iota auto-increment is interrupted, you have to explicitly restore it

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const (
	A1 = iota // 0
	B1        // 1
	C  = "c"  // c
	D         // c
	E  = iota // 4
	F         // 5
)

how iota got applied into the const enum, the pesudo code is as such :

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for iota, spec := range ValueSpecs {
 for i, name := range spec.Names {
 obj := NewConst(name, iota...) // pass in iota
 ....
 }

from this code we can see that, even iota appears multiple times in a single line, it won’t auto-increment