What Is the Equivalent of Console. Log in Pine-Script?

NOTES

1. While this answer's content is not deprecated, the Pine User Manual now contains a page on debugging techniques explaining much of this answer's content, and more.
2. RicardoSantos has a very nice DebugConsole library which v5 users can access.

There is actually something similar to a console for Pine developers; it's the Data Window. We use it constantly to debug. The idea is to use plotchar() in this way:

plotchar(bar_index, "Bar Index", "", location = location.top)

This will not disrupt the indicator's scale and won't print anything in it, but it will show a value in the Data Window, as is explained in the second question of the PineCoders FAQ's section on debugging. As you move your mouse over chart bars, the corresponding value of the variable/expression will show in the Data Window. The FAQ explains other useful techniques that can be used to debug on the chart, as that is sometimes more efficient.

We use an AutoHotkey macro that creates the required plotchar() statement from a variable or expression previously copied to the clipboard. This is the AHK macro:

^+C:: SendInput plotchar(^v, "^v", "", location.top){Return}

The Data Window is also a great option as a display panel for scripts requiring the display of many values, such as our Backtesting & Trading Engine, which makes extensive use of it:

With pine v.4 there's new way to print text. You can use labels for that:

//@version=4
study("Print text values", overlay=true)
x = bar_index
y = close
txt = tostring(close)
label.new(x, y, txt) // print value of close

If you just want to print a single value (not at every bar), you can do:

if (barstate.islast)
    label.new(bar_index, 0, "Your value here, ex: " + syminfo.tickerid)

What I do is, I use table to display the values i want to display. And this works like magic in replay bar. Here's an example:

//@version=4
study("My Script", overlay=true)

sma20 = sma(close, 20)
text = "sma 20: " + tostring(sma20)

tableColumn = 1
tableRow = 1
var table panel = table.new(position.top_right, tableColumn, tableRow)
if barstate.islast
    table.cell(panel, 0, 0, text, bgcolor=color.black, text_color=color.white)

And here's the result:

As mentioned by not2qubit, there is technically not a way to print stuff to the console of TradingView or so.

But we can create labels to "print" stuff, which is why I wrote this little function.

It will "print" the text you feed it on the latest bar_index. If you print multiple things, the labels will be stacked on top of each other.

Only tested on PineScript Version 5

var global_print_counter = array.new_int()
array.push(global_print_counter, 0)
print(string txt = "") => 
    if txt != "" and barstate.islast
        int print_counter = array.get(global_print_counter, 0)
        printLabel = label.new(x=bar_index, y=high + (print_counter * 75), textcolor=color.white, color=color.black, text=txt)
        array.set(global_print_counter, 0, print_counter + 1)

Example:

print("Hello World!")
print("Hello World, again!")