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Overview

Crossover functions compare two sources at the current and previous bar to detect directional changes. They all return a boolean.
All crossover functions require a source function (a (offset?) => number | null callable), not a scalar. Pass ctx.close or input.source() - not ctx.close().

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ta.crossover(a, b) - A crosses above B

Returns true when a was below b on the previous bar and is now above b. 
onBar(() => {
  const fast = ta.ema(ctx.close, 9);
  const slow  = ta.ema(ctx.close, 21);

  // We need series for crossover detection
  // ta.crossover works when a and b are source functions
  // Here we pass inline functions that reference our computed values:
  const fastSrc = Series("fast-ema-src");
  const slowSrc = Series("slow-ema-src");

  const prevFast = fastSrc.get(1);
  const prevSlow = slowSrc.get(1);
  fastSrc.set(fast);
  slowSrc.set(slow);

  const crossed = !na(prevFast) && prevFast < prevSlow && fast > slow;
  plot("Cross", crossed ? ctx.low() * 0.998 : NaN, { style: "arrowup", colorUp: "#22c55eFF", size: 8 });
});
The idiomatic form passes source functions directly: 
onBar(() => {
  // ta.crossover accepts source functions directly
  const bullCross = ta.crossover(ta.ema.bind(null, ctx.close, 9), ctx.close);
  plot("Signal", bullCross ? 1 : NaN);
});

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ta.crossunder(a, b) - A crosses below B

Returns true when a was above b on the previous bar and is now below b. 
onBar(() => {
  const rsi = ta.rsi(ctx.close, 14);
  // Detect RSI crossing below overbought level
  // b can be a number constant
  const crossed = ta.crossunder(() => rsi, 70);
  plot("OB Exit", crossed ? ctx.high() * 1.001 : NaN);
});

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ta.cross(a, b) - Either direction

Returns true for either crossover or crossunder. 
onBar(() => {
  const crossed = ta.cross(() => ta.rsi(ctx.close, 14), 50);
  bgcolor(crossed ? "#f59e0b20" : null);
});

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ta.rising(source, length) - Rising over N bars

Returns true if source(0) > source(length) - current value is higher than length bars ago. 
onBar(() => {
  const isRising = ta.rising(ctx.close, 5);
  bgcolor(isRising ? "#22c55e10" : null);
});

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ta.falling(source, length) - Falling over N bars

Returns true if source(0) < source(length). 
onBar(() => {
  const isFalling = ta.falling(ctx.close, 5);
  bgcolor(isFalling ? "#ef444410" : null);
});

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Source type requirement

The a and b arguments to crossover/crossunder/cross must be source functions ((offset?) => number | null). A plain number is accepted for b only. Passing a scalar to a causes a TypeError. 
// Correct: ctx.close is a function
ta.crossover(ctx.close, ctx.open);

// Correct: b can be a number constant
ta.crossunder(() => ta.rsi(ctx.close, 14), 70);

// Wrong: passing scalars
// ta.crossover(ctx.close(), ctx.open()); // TypeError

Moving averages

Common sources for crossover signals.

Oscillators

RSI crossovers (crossing 70/30) are a classic use case.

Series

Manual crossover detection with Series for full control.