Module 04

Branching

Bitcoin Script has no loops and no goto — but it does have conditionals. OP_IF lets a script take different paths based on a value on the stack, enabling scripts that respond to their inputs.

0 / 4 sections

What you'll learn

→Write OP_IF / OP_ELSE / OP_ENDIF blocks
→Use OP_NOTIF for inverted conditions
→Explain how the condition stack tracks nested branches
→Apply OP_VERIFY and OP_RETURN for guard clauses

OP_IF / OP_ELSE / OP_ENDIF

OP_IF pops the top item. If it's truthy, the block between IF and ELSE executes. If falsy, the ELSE block executes (if present). OP_ENDIF closes the block. This is the only form of branching in Bitcoin Script.

The condition value is consumed — it's popped from the stack before any branch executes.

OP_IF (true branch)

Condition is 1 (truthy) → the IF branch runs, pushing 7.

Script

▶OP_1

OP_IF

OP_7

OP_ELSE

OP_9

OP_ENDIF

Stack0 items

↑ top of stack

nothing here yet

press Step or Run to push an item

bottom

OP_1—Pushes the number 1 onto the stack.

0 stepsReady

OP_IF (false branch)

Condition is 0 (falsy) → the ELSE branch runs, pushing 9.

Script

▶OP_0

OP_IF

OP_7

OP_ELSE

OP_9

OP_ENDIF

Stack0 items

↑ top of stack

nothing here yet

press Step or Run to push an item

bottom

OP_0—Pushes an empty byte array (false / zero) onto the stack.

0 stepsReady

Notice the opcodes in the skipped branch don't execute and don't touch the stack — the interpreter jumps over them entirely. The internal condition stack tracks which level is active.

✦Real Bitcoin

Branching in Lightning

Hash Time-Locked Contracts (HTLCs) — the mechanism behind every Lightning payment — use OP_IF to encode two settlement paths: reveal a preimage or wait for a timeout. Branching is what makes multi-hop routing safe.

OP_NOTIF and nesting

OP_NOTIF is the mirror image of OP_IF: it executes its block when the condition is falsy. Conditionals can be nested — each OP_ENDIF closes the innermost open block.

OP_NOTIF

Condition is 0 (falsy) → the NOTIF block runs, pushing 5.

Script

▶OP_0

OP_NOTIF

OP_5

OP_ENDIF

Stack0 items

↑ top of stack

nothing here yet

press Step or Run to push an item

bottom

OP_0—Pushes an empty byte array (false / zero) onto the stack.

0 stepsReady

Nested OP_IF

Two nested conditionals. Both conditions are true → innermost IF branch runs, pushing 16.

Script

▶OP_1

OP_IF

OP_1

OP_IF

OP_16

OP_ELSE

OP_15

OP_ENDIF

OP_ELSE

OP_0

OP_ENDIF

Stack0 items

↑ top of stack

nothing here yet

press Step or Run to push an item

bottom

OP_1—Pushes the number 1 onto the stack.

0 stepsReady

OP_VERIFY and OP_RETURN

OP_VERIFY pops the top item and immediately fails the entire script if it's falsy. It's the assertion opcode — used when you need to guarantee a condition is true before proceeding. Many opcodes end with VERIFY as a suffix (e.g. OP_EQUALVERIFY) — these are shorthand for running the base opcode then verifying the result.

OP_RETURN unconditionally fails the script and marks the output as unspendable. It's used in data-carrier outputs for embedding metadata.

OP_VERIFY (pass)

5 == 5 → OP_VERIFY passes → OP_1 pushes true. Script succeeds.

Script

▶OP_5

OP_5

OP_EQUAL

OP_VERIFY

OP_1

Stack0 items

↑ top of stack

nothing here yet

press Step or Run to push an item

bottom

OP_5—Pushes the number 5 onto the stack.

0 stepsReady

OP_VERIFY (fail)

5 ≠ 6 → OP_VERIFY fails the script immediately.

Script

▶OP_5

OP_6

OP_EQUAL

OP_VERIFY

OP_1

Stack0 items

↑ top of stack

nothing here yet

press Step or Run to push an item

bottom

OP_5—Pushes the number 5 onto the stack.

0 stepsReady

Experiment: what happens if you put data after an OP_RETURN? Try OP_1 OP_RETURN OP_2 below.

Ctrl+Enter to run

Script

▶OP_1

OP_RETURN

OP_2

Stack0 items

↑ top of stack

nothing here yet

press Step or Run to push an item

bottom

OP_1—Pushes the number 1 onto the stack.

0 stepsReady

Your turn

Challenge

Conditional result

Write a script that uses OP_IF to leave 42 on the stack when the condition is true, and anything else when false. Make the true branch execute.

Ctrl+Enter to check

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